Nitric acid is used for manufacturing nitrogen fertilizers and organic fertilizers, in the form of ammonium nitrate, potassium nitrate, or nitro phosphate. Nitric acid is also used in explosives, the plastics industry, and the chemical industry.
Nitric acid (HNO3) is manufactured by catalytic combustion of ammonia to produce nitrogen monoxide. The process is typically conducted in three steps. The first step is oxidation of ammonia. The oxidation of ammonia takes place in the presence of a catalyst with yields ranging from 93-98% depending on the operating conditions used.
The second step is the condensation of the water of combustion and oxidation of the nitrogen monoxide. The second step may yield nitrogen dioxide, nitrogen tetroxide, or a mixture of the two. The third step involves the absorption or fixation of the nitrogen dioxide and nitrogen tetroxide. The reaction intermediates and products of the first and second steps may be commercially viable in their own right. Thus, the first and second steps may be used to form various products, even if nitric acid is not desired.
The catalyst used in the first step of the ammonia oxidation process is generally in the form of precious metal gauze. Because of the high temperatures that result from the oxidation of ammonia, the precious metal gauze often fractionates and particles thereof become entrained in the gaseous material present in the reaction vessel. This “metal loss” generally increases with the severity of reaction conditions. For example, the amount of metal loss generally increases as the temperature and the rate of production increases.
This metal loss has a substantial monetary impact on the economic feasibility of the ammonia oxidation process due to the high cost of the precious metal gauze. Furthermore, the metal loss may result in the poisoning of downstream catalytic material utilized in subsequent reaction vessels which are downstream of the ammonia oxidation reactor.
One solution to the problem of metal loss has been to provide an external filter housing and filtration device downstream from the ammonia oxidation reaction vessel. These external filter housing configurations cause pressure drop and require additional space in an already crowded plant facility. Another solution has been the use of fibrous material in a location immediately adjacent to the precious metal gauze. Unfortunately, the use of such fibrous materials in this location does not achieve efficient filtration and requires a confined space entry procedure in order to service the fibrous material during the requisite periodic maintenance.